searches for dark matter at cms and...
TRANSCRIPT
Searches for dark matter
at CMS and ATLAS
K. BierwagenJohannes Gutenberg University Mainz
On behalf of the ATLAS and CMS Collaboration
March 20, 2018
20.03.2018 K. Bierwagen - Moriond QCD 2018 2
Introduction
Astrophysical observations point to the existence ofDark Matter (DM)
Rotation curves Bullet Cluster CMB
Underlying nature of DM remains unknown
WIMP paradigm → Motivation to consider collider searches for DM
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Dark Matter Signatures
Mono-photon Mono-jet
Mono-Higgs
Mono-W/Z
Mono-t/tt
A wide range of finalstates can be investigated
with the LHC
Mono-b/bb
For more details on dijetsearches see talks by
C. Gwilliam and D. Moran
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Missing Transverse Energy
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Search for DM with jets
Jet pT of 973 GeV
balanced by a ET
miss of 954 GeV
Largest ET
miss+X cross-section
at LHC
MET
signal
background
Search for high MET excess
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Signal Region
CMS-EXO-16-048
JHEP 01 (2018) 126
> 0.4Up to 3additional jets(p
T>30 GeV)
Jet pT>250 GeV
|η|<2.4
MET > 250 GeV
e & μ veto
N.B. ATLAS selection for illustration purpose
Crucial to measure SMbackground precisely
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Backgrounds
● Main Backgrounds
– Z(vv)+jets: irreducible
– W(lv)+jets: irreducible whenleptons are out of detectoracceptance
● Backgrounds constrained indedicated control regions
● V+jet modeled at NNLO QCD& NLO EW– Includes theory predictions via
MC reweighting based on pT(V)
57% 38%
6%Other
Z+jets W+jets
72%
23%5%Other
Z+jets
W+jets
High METLow MET
59%37%
4%Other
Z+jetsW+jets
84%
11%6%
Other
Z+jets
W+jets
ATLAS
CMS
Lindert, J.M., Pozzorini, S., Boughezal, R. et al. Eur. Phys. J. C(2017) 77:829
Mono-jet
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Analysis Strategy (ATLAS)
Z(μμ)+jets
W(μν)+jets (b-veto) W(eν)+jets
W(μν)+jets (b-tag)
SR
Simultaneous fit of control and signal regions to extract result
No significant excess above SM backgrounds observed
JHEP 01 (2018) 126
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Analysis Strategy (CMS)
SRZ(μμ)+jets
W(eν)+jetsW(μν)+jets
Z(ee)+jets
γ+jets
CM
S-E
XO
-16-
0 48
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● Axial-vector and vector models are excluded for mediator massesup to 1.55 TeV to 1.8 TeV for very low DM masses
DM Interpretations
JHEP 01 (2018) 126
CMS-EXO-16-048
Axial-vector Mediator Vector Mediator
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DM Interpretations
Scalar colored mediator
● Jet either from ISR or frommediator decay
● Mediator has spin 0, couples tofirst two generations of quarks
● 3 free parameters: mη, m
χ, g
● Minimal mediator width Mediator masses excluded up to1.7 TeV for very low DM masses
JHEP 01 (2018) 126
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Searches for DM with Higgs bosons
● Higgs may mix with new dark mediators● H → bb is ~ 58% of BR(H→all), large backgrounds● H → ττ is ~ 6% of BR(H→all), lower background● H → γγ is ~ 0.2% of BR(H→all), very clean final state
Z'-2HDM model
● Baryonic Z' models excluded up to 815 GeVfor m
χ=1 GeV
● MZ' excluded up to 2.6 TeV and m
A up to 0.6
for Z'-2HDM models
Z'B model
Phys. Rev. Lett. 119 (2017) 181804
CMS-PAS-EXO-16-055
NEW
For more details see alsothe talk by D. Moran
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Combination of Searches
DM parameter space is being probed ever more thoroughly
Dijet searches can exclude mediator masses between 50 GeV and2.7 TeV for almost whole DM mass range
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Comparison with direct detection experiments
Spin-independent DM-nucleon cross-section
Spin-dependent DM-proton cross-section
● Collider searches complements DD experiments for DM massesbelow 5 GeV for spin-independent DM-nucleon cross-section
● Strong limits for spin-dependent DM-proton cross-section, increasedsensitivity by 3 orders of magnitude
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Summary
● DM searches at the LHC remain a thriving field of research
● A large number of mono-X searches have been performedby ATLAS and CMS, already probing a large part of theparameter space– Mono-jet is one of the most powerful channels
● So far no evidence for physics beyond the SM has beenfound
● LHC searches complementary to direct searches
● LHC will provide plenty of new data in the coming years– Stay tuned!
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Back-Up
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Large Hadron Collider
The LHC
Lake of GenevaInstalled in 26.7 km LEP tunnelDepth of 70 – 140 m
ATLAS
CMS
CMS
● ATLAS and CMS ● General-purpose detectors, covering ~ full solid angle ● Excellent tracking, calorimetry, muon spectrometer
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DM Interpretations
● Pseudoscalar models are excluded for mediator masses up to 400GeV and DM masses up to 150 GeV
Pseudoscalar MediatorScalar Mediator
CMS-EXO-16-048 CMS-EXO-16-048
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Searches for DM with jets
● Hadronic W/Z decays more common● BR(W → qq) ≈ 3 × BR(W → lν) ● BR(Z → qq) ≈ 10 × BR(Z → ll)
● Makes use of jet substructure ● Primary background: V+jets
● Z (→ νν) + jet, jet fakes W/Z decay Phys. Lett. B 763 (2016) 251
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● Very clean compared to ET
miss+jet ● However, much smaller cross-section
● Signature: large missing transverse energy recoiling against high energetic photon
● Main backgrounds: Z+γ and W+γ
● Vector and axial-vectormodels are excluded up to1.2 TeV for low DM masses
Searches for DM with photons
Eur. Phys. J. C 77 (2017) 393
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Searches for DM with leptonically decaying Z
● An even cleaner final state ● However, further reduced statistics
● Signature: large missing transverseenergy recoiling against two isolatedleptons
● Primary background: VV
● Axial-vector models areexcluded up to 560 GeV forlow DM masses
PLB 776 (2017) 318
CMS-EXO-16-052
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Searches for DM with heavy quarks
● Signature: large missing transverseenergy and boosted heavy quarksdecaying leptonically andhadronically
● Main backgrounds:
– Z+jets, W+jets (low ET
miss)
– tt, Z+jets (high ET
miss)
Eur. Phys. J. C 78 (2018) 18
CMS-EXO-16-051
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Dijet Searches
● DM mediator couples to SM particleand hence can also decay into SMparticles
● Search for new resonances in dijetinvariant mass spectrum
● Standard dijet search: ● Sensitive to m
Z′≥1.5 TeV
● Can we circumvent the triggerbarrier?● Trigger-level search:
● Dedicated data stream allows to godown to 450 GeV
● Dijet+ISR search: ● Trigger on ISR jet or ISR photon ● ISR jet: mediator mass of ≥350 GeV
● ISR γ: mediator mass of ≥200 GeV
Phys. Rev. D 96 (2017) 052004
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Dijet Searches
● Different dijet searches sensitive to different mass regimes● Analyses are working together to probe dark mediator
parameter space ● Combination assumes a lepto-phobic axial-vector mediator
JHEP 01 (2018) 097
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Combination of Searches
Big picture changes with the choice of couplings
● Dijet and mono-X constraints weakened if gq=0.25 → 0.1
● Strong constraints from dilepton search if gl>0